A Google search for “church membership” unearths some interesting
statistics: In the USA, only 15% to 20% of the population are members of a
church. These figures are lower in Canada and even lower in Europe.

Given these figures, why are churches exempt from property taxes? Why
should the unchurched support the churched through property taxes?

I think that all church property should be taxed. The tax could be phased
in over ten years so that the churches could have time to adjust their
budgets.